There is a trend across states to mandate increasing amounts of energy to be derived from renewable resources. Electric utilities in California, for instance, are required to have 33% of their retail sales derived from eligible renewable energy resources by 2020, and this requirement may later increase to 50%.
The challenges of integrating major renewable energy resources, such as wind and solar energy, into the electric distribution grid are well known. These challenges include intermittency, low predictability, and a need for fossil-fuel powered backup generation, for example. Such challenges with renewable energy resources make it difficult to match electric power demand and supply, which is very important for both users and producers of electrical energy.
Storage systems for temporarily storing energy to be available to supply to the electrical grid are known. As examples, these may take the forms of pumped water storage, flywheel storage, chemical storage such as storage in batteries, as well as compressed air energy storage (CAES). Each system has its own requirements and inefficiencies. Pumped water storage must have access to large amounts of water and a suitable elevation change. CAES must have access to a storage facility, as well as the systems necessary for compressing and expanding the air efficiently.
Conventional CAES technology uses relatively cheap electric power during times of low demand to compress air into a reservoir where it is stored at high pressure. During times of high demand, when the price of electric power is relatively expensive, the stored air is expanded through a turbine with fuel that is ignited inside the turbine to generate power. The stored air may also be heated prior to entry into the gas turbine.
Also, conventional CAES is an open loop system in that the air used for compression is typically pulled from the atmosphere. Then, after expansion the air is vented back to the atmosphere. CAES has not proliferated in the United States due to its high initial cost for storage reservoir excavation and for its specialized expansion equipment. Also, CAES economics rely on the price spread between electricity prices at peak and low demand hours to make money.
Additionally, it has not been demonstrated that conventional CAES systems can safely use depleted natural gas reservoirs to store the compressed air. This can be extremely dangerous since the natural gas left in the reservoir, or that may still be seeping into the reservoir from natural sources, could form a flammable mixture.
Embodiments of the invention address shortcomings and other issues in the prior art.